In an internal combustion engine, engine valve actuation is required in order to produce positive power, and may also be used to produce engine braking and/or exhaust gas recirculation (EGR). During positive power, one or more intake valves may be opened to admit air into a cylinder for combustion during the intake stroke of the piston. One or more exhaust valves may be opened to allow combustion gases to escape from the cylinder during the exhaust stroke of the piston.
One or more exhaust valves may also be selectively opened to convert, at least temporarily, the engine into an air compressor for engine braking operation. This air compressor effect may be accomplished by either opening one or more exhaust valves near piston top dead center (TDC) position for compression-release type braking, or by maintaining one or more exhaust valves in a relatively constant cracked open position during much or all of the piston motion, for bleeder type braking. In either of these methods, the engine may develop a retarding force that may be used to help slow a vehicle down. This braking force may provide the operator with increased control over the vehicle, and may also substantially reduce the wear on the service brakes. Compression-release type engine braking has been long known and is disclosed in Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference.
One proposed method of adjusting valve timing and lift to selectively provide engine braking, given a fixed cam profile, has been to incorporate a “lost motion” device in the valve train linkage between the engine valve and the cam that provides the engine braking motion. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage assembly. In a lost motion system, a cam lobe may provide the “maximum” (longest dwell and greatest lift) motion needed for an engine valve event, such as engine braking. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve (e.g., for engine braking), and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application and which are incorporated herein by reference.
In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may include a solenoid trigger valve in communication with the hydraulic circuit that includes the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit when the master piston is acted on by certain of the cam lobes. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the hydraulic fluid displaced by the motion of the master piston, which reciprocates in response to the cam lobe acting on it. When the solenoid is opened, the circuit may drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston and the engine valve.
The braking power of a compression-release type engine brake may be increased by selectively actuating the exhaust valves to carry out brake gas recirculation in combination with compression release braking. Brake gas recirculation (BGR) can be accomplished by opening an exhaust or auxiliary valve near bottom dead center of the intake or expansion stroke of the piston and keeping the exhaust or auxiliary valve open during the first portion of the exhaust or compression stroke of the engine. Opening the exhaust or auxiliary valve during this portion of the engine cycle may allow exhaust gas to flow into the engine cylinder from the relatively higher pressure exhaust manifold. The introduction of exhaust gases from the exhaust manifold into the cylinder may increase the total gas mass and gas pressure in the cylinder at the time of the immediately following compression-release event. This increased gas mass and pressure in the engine cylinder may increase the braking power produced by the compression-release event.
There are many different systems that may be used to selectively actuate an exhaust or auxiliary valve to produce BGR and compression-release events. One known type of actuation system is a lost motion system, described in the aforenoted Cummins patent. An example of a lost motion system and method used to obtain engine braking and brake gas recirculation is disclosed in Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) which discloses a method of conducting brake gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the intake stroke, and which is hereby incorporated by reference. Gobert uses a lost motion system to enable and disable compression-release braking and brake gas recirculation. The system disclosed in Gobert opens the exhaust valve near bottom dead center of the intake stroke for a BGR event, closes the exhaust valve before the midway point of the compression stroke to terminate the BGR event, and opens the exhaust valve again near top dead center of the same compression stroke for a compression-release event. As a result, the exhaust valve actuated in accordance with the Gobert system must be rapidly seated and unseated between the BGR and compression-release events.
In many internal combustion engines, the intake and exhaust valves may be actuated by fixed profile cams, and more specifically, by one or more fixed lobes that are an integral part of each cam. The cams may include a lobe for each valve event that the cam is responsible for providing. The size and shape of the lobes on the cam may dictate the valve lift and duration which result from the lobe. For example, an exhaust cam profile for a system constructed in accordance with the aforenoted Gobert patent may include a lobe for a BGR event, a lobe for a compression-release event, and a lobe for a main exhaust event.
Compression-release engine braking is not the only type of engine braking known. The operation of a bleeder type engine brake has also long been known. During bleeder type engine braking, in addition to the normal exhaust valve lift, the exhaust valve(s) may be held slightly open continuously throughout the remaining engine cycle (full-cycle bleeder brake) or during a portion of the cycle (partial-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the exhaust valve is closed for the former during most of the intake stroke.
Usually, the initial opening of the braking valve(s) in a bleeder braking operation is far in advance of the compression TDC (i.e., early valve actuation) and then lift is held constant for a period of time. As such, a bleeder type engine brake may require much lower force to actuate the valve(s) due to early valve actuation, and generates less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake. Moreover, bleeder brakes often require fewer components and can be manufactured at lower cost. Thus, an engine bleeder brake can have significant advantages.
Some lost motion system used for engine braking may utilize a dedicated cam lobe to actuate a rocker arm to perform engine braking and/or some other engine valve actuation. Examples of such systems are disclosed in U.S. Pat. Nos. 7,392,772 and 5,975,251, which are incorporated by reference herein. In dedicated cam engine braking systems, it may be desirable to maintain a lash space between the cam and the rocker arm used to actuate the engine valve for engine braking when the engine is not providing engine braking (i.e., during positive power operation of the engine). U.S. Pat. Nos. 7,392,772 and 5,975,251 both disclose mechanisms for biasing a rocker arm away from a dedicated engine braking cam lobe during positive power. The biasing mechanisms disclosed in the foregoing patents, however, both require that hydraulic fluid passages be provided in the rocker arms themselves. Providing hydraulic passages within rocker arms, and supplying such passages with hydraulic fluid may be difficult and add expensive to an engine braking system.
Accordingly, it is an advantage of some, but not necessarily all, embodiments of the present invention to provide non-hydraulic means for biasing a rocker arm away from a dedicated cam, and/or to provide a hydraulic means for biasing a rocker arm away from a dedicated cam wherein the hydraulic means is not incorporated into a rocker arm. Additional advantages of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.